1. Field of the Invention
The present invention relates to a solid oxide fuel cell (SOFC) comprising a solid electrolyte layer placed between an air electrode layer and a fuel electrode layer, and is also called a solid electrolyte type fuel cell.
2. Description of the Related Art
A solid oxide fuel cell comprising a layered structure, in which a solid electrolyte layer composed of an oxide ion conductor is interposed between an air electrode and a fuel electrode layer, has been developed as a fuel cell for novel power generation. Solid oxide fuel cells are classified roughly into two types, such as a cylindrical type as shown in FIG. 9A and a planar type as shown in FIG. 9B.
The cylindrical type cell shown in FIG. 9A comprises an isolated porous ceramic cylinder substrate 1, an air electrode layer 2, a solid electrolyte layer 3, and a fuel electrode layer 4. The air electrode layer 2, the solid electrolyte layer 3, and the fuel electrode layer 4 are adhered onto the outer surface of the isolated porous ceramic cylinder substrate 1, in turn, so as to be arranged concentrically with each other. A conductive inter connector 5, which is a terminal of the air electrode, is laminated onto the solid electrolyte layer 3 so as to connect with the air electrode layer 2 via the solid electrolyte layer 3 and so as not to contact the fuel electrode layer 4. These layers may be formed by spray coating method, electrochemical deposition method, or slip casting method, etc.
The planar type cell shown in FIG. 9B comprises a solid electrolyte layer 3, an air electrode layer 2 laminated on one side of the solid electrolyte layer 3, and a fuel electrode layer 4 laminated on the other side of the solid electrolyte layer 3. The planar type cell is used by connecting another planar type cell via a dense inter connector 5 comprising gas channels on both sides. The planar type cell is formed by sintering a green sheet formed either by the doctor blade method, or extension method, or the like, thereby forming the solid electrolyte layer 3 which in turn coated by a slurry for the air electrode layer 2 on one side and a slurry for the fuel electrode layer 4 on the other side of the sheet. The final sintering can be done all together or in sequence. Moreover, the planar type cell can also be formed by preparing green sheets of the solid electrolyte layer 3, the air electrode layer 2 and the fuel electrode layer 4, superposing, and sintering them all together. Such a wet method must be low cost. Also, the spraying method or electrochemical deposition method can be used, similar to the case of the cylindrical type cell.
In these solid oxide fuel cells, oxygen is supplied to the air electrode layer side, and fuel gas, such as H2 and CO, is supplied to the fuel electrode layer side. The air electrode layer 2 and the fuel electrode layer 4 are made of a porous material so as to allow gases to diffuse to the interface between the solid electrolyte layer 3 and the air electrode layer 2 or the fuel electrode layer 4. Oxygen supplied to the air electrode layer side passes through pores of the air electrode layer 2, and reaches in the vicinity of the interface between the air electrode layer 2 and the solid electrolyte layer 3. Then, the oxygen receives electrons from the air electrode layer 2, to be ionized, (O2−). The oxide ions diffuse toward the fuel electrode layer 4 through the solid electrolyte layer 3. When the oxide ions reach in the vicinity of the interface between the solid electrolyte layer 3 and the fuel electrode layer 4, the oxide ions react with the fuel gas, generate a reaction product, such as H2O and CO2, and discharge electrons to the fuel electrode layer 4.
The solid electrolyte layer 3 functions as a partition wall to prevent direct contact between the fuel gas and air, while being a medium for conducting oxide ions. Therefore, the solid electrolyte layer 3 must have gas impermeability and a high density. Moreover, the solid electrolyte layer 3 must be made of a material which has a high oxide ionic conductivity, a high chemical stability under the oxidizing atmosphere at the air electrode side and the reducing atmosphere at the fuel electrode side, and a high degree of being thermally shock-proof. For example, yttria stabilized zirconia (YSZ) is generally used as the material for the solid electrolyte layer 3.
However, the stabilized zirconia has a problem of decreasing ionic conductivity when the temperature decreases. For example, the ionic conductivity of Y2O3 stabilized zirconia is 10−1 s/cm at 1000° C., and is 10−4 s/cm at 500° C. Therefore, a fuel cell comprising a solid electrolyte layer 3 made of such electrolyte material must be used at temperatures about 1000° C., or at least 800° C. That is, the fuel cell must be used at high temperatures.
Japanese Unexamined Patent Application, First Publication No. Hei 11-335164 discloses an oxide ionic conductor having a perovskite structure as a material which can solve such a problem. The oxide ionic conductor is represented by general formula:                Ln1−xAxGa1−y−zB1yB2zO3, wherein Ln indicates lanthanide rare-earth metals, A indicates alkaline earth metals, B1 indicates non-transition metals, and B2 indicates transition metals. Namely, the oxide ionic conductor is a multiple oxide of 5 components (Ln+A+Ga+B1+B2) which is obtained by doping 3 kinds of elements, e.g. an alkaline earth metal (A), a non-transition metal (B1), and a transition metal (B2) into a lanthanide-gallate (LnGaO3), or 4 components (Ln+A+Ga+B2) which is obtained by doping 2 kinds of elements, e.g. an alkaline earth metal (A) and a transition metal (B2) into a lanthanide-gallate (LnGaO3).        
The relationship between the percentage of B2 which are transition metal elements doped in the B site, and the total electric conductivity and the ionic transference number, in 5 components multiple oxide ionic conductor (e.g. La0.8Sr0.2Ga0.8Mg0.2−cCocO3) is shown in FIG. 2. Total electric conductivity shown in FIG. 2 contains both the ionic and electronic components. It is clear that this lanthanide-gallate oxide has a high oxide ionic conductivity for a wide range of temperatures, which higher than that of stabilized zirconia, and also has high heat resistance. Furthermore, it is also confirmed that lanthanide-gallate oxide has a high ionic transference number at all oxygen partial pressures from the oxygen atmosphere to the hydrogen atmosphere. In other words, it is clear from FIG. 2 that the percentage of the oxide ionic conductivity with respect to the total electric conductivity is remarkably high in lanthanide-gallate oxide, which acts as an electronic-ionic mixed conductor. Therefore, the operating temperature limit of a solid oxide fuel cell, which is about 1000° C. in general, can be lowered by using lanthanide-gallate oxide for the solid electrolyte layer 3.
In order to increase the efficiency of the fuel cell, it is necessary to prevent electrons, which were discharged in the fuel electrode by reacting oxide ions with the fuel, from returning to the air electrode layer through the solid electrolyte layer, and to catch the electrons securely in the fuel electrode. To achieve this, the ionic transference number of the solid electrolyte comprising the solid electrolyte layer should ideally be 1.0. In other words, it is preferable that the total electric conductivity of the solid electrolyte is entirely due to the oxide ions, and that the electronic conduction is not possible between the air electrode layer and the fuel electrode layer at all. In order to bring the ionic transference number of lanthanide-gallate oxide disclosed in the Japanese Unexamined Patent Application, First Publication No. Hei 11-335164 close to 1.0, an added amount of a transition metal (B2z), namely Co, is needed to cause a decrease, as shown in FIG. 2.
However, when the added amount of Co is small, the total electric conductivity is low and the performance of the fuel cell deteriorates.
When the thickness of the solid electrolyte layer 3 significantly decreases, the total electric conductivity increases, and the problem may be solved. However, when the thickness of the solid electrolyte layer 3 decreases, the partition wall function, which functions so as to prevent direct contact between the fuel gas and air, may be decreased. Therefore, there is a limit in the thickness of the solid electrolyte layer.
In consideration of the above-described problems with conventional technology, one of the objectives of the present invention is to provide a solid oxide fuel cell which has an improved efficiency achieved by a solid electrolyte layer having improved ionic conductivity, while maintaining the partition wall function.